The present invention relates to a fuel cell, and more specifically, to a fuel cell having an electrolyte matrix layer impregnated with an electrolyte solution (e.g., phosphoric acid solution).
Generally, in fuel cells, an easily oxidizable fuel gas (e.g., hydrogen gas) and an oxidant gas (e.g., oxygen gas) having oxidizing capability are subjected individually to electrode reactions. By these reactions, free energy of Gibbs is released and taken out as electric power. These fuel cells are advantageous in having high power-generation efficiency and in being pollution-free.
One such prior art fuel cell comprises a number of unit cells, for use as minimum generator elements, and separator plates electrically connecting the unit cells. In the fuel cell of this type, the unit cells are stacked in layers, and the separator plates are interposed individually between the adjacent unit cells. This configuration is necessary because a number of unit cells must be arranged in series, in order to enable the fuel cell to produce a high electromotive force, since each unit cell can produce an electromotive force of only 1 V or less.
Each unit cell includes an anode and a cathode, each formed of a porous substrate, and an electrolyte matrix layer interposed between them. The matrix layer is impregnated with an electrolyte solution of a very high concentration, e.g., 95-% phosphoric acid solution. An anode catalyst layer is formed on the matrix-side surface of the anode, while a cathode catalyst layer is formed on the matrix-side surface of the cathode.
The fuel cell is further provided with a fuel gas channel, used to feed the fuel gas to the anode, and an oxidant gas channel through which the oxidant gas is fed to the cathode. Depending on the locations of these channels, fuel cells are generally classified into the following three types. A first one is a so-called bipolar type, in which a fuel gas channel and an oxidant gas channel are formed individually in two opposite surfaces of a separator plate. A second one is a so-called ribbed-substrate type, in which a fuel gas channel is formed in the separator-side surface of an anode, and an oxidant gas channel is formed in the separator-side surface of a cathode. A third one is a so-called hybrid type, in which a fuel gas channel is formed in the separator-side surface of an anode, and an oxidant gas channel is formed in the cathode-side surface of a separator. Fuel cells of the hybrid type are commonly used, in view of the strength of the stacked structure and the ease of diffusion of the oxidant gas.
If the fuel gas and oxidant gas are delivered to the anode and cathode in each unit cell, and brought into contact with the anode and cathode catalyst layers, respectively, the following electrode reactions take place: EQU H.sub.2 .fwdarw.2H.sup.+ +2e (1)
on the anode side, and EQU 1/2O.sub.2 +2H.sup.+ +2e .fwdarw.H.sub.2 O (2)
on the cathode side. During these reactions, H.sup.+ in the electrolyte solution, with which the electrolyte matrix layer is impregnated, functions as a medium for charge transfer.
After the electrode reactions, the fuel and oxidant gases are discharged through their respective channels. With the progress of the electrode reaction, as indicated by formula (2), water is produced. The water tends to lower the concentration of the electrolyte solution. In order to prevent the concentration reduction, the water is discharged quickly with the oxidant gas. At the same time, however, some of the electrolyte solution is discharged, so that the solution in the electrolyte matrix layer is reduced. Thus, the matrix layer increases its resistance, thereby lowering the voltage of the fuel cell.
In the prior art fuel cells, therefore, if the electrolyte solution is reduced, the electrolyte matrix layer is replenished with electrolyte solution, to prevent the fuel cell voltage from dropping.
In the fuel cells of the popular hybrid type, the fuel gas channel of the anode is defined by a plurality of porous ribs, which have been previously impregnated with the electrolyte solution. If the electrolyte solution in the electrolyte matrix layer lessens, the solution in the ribs is delivered to the matrix layer, after permeating the porous anode. Thus, the matrix layer is replenished with the electrolyte solution, thereby maintaining the quantity of the solution therein, so that the voltage of the fuel cell is prevented from dropping. If all the voids of the porous anode are filled up with the electrolyte solution, however, the fuel gas cannot reach its reaction point. Accordingly, the electrolyte solution is reserved only in about 60% of the voids of the anode. In the conventional fuel cells, therefore, the electrolyte solution cannot be resupplied to a level high enough to permit a continuous operation of the fuel cell for 40,000 hours, which is a commercial power-generating time.
Moreover, a bank portion is formed on either side of the fuel gas channel of the anode. It is formed with a channel, capable of storing an electrolyte solution, for use as an alternative solution resupply means. During operation of the fuel cell, the electrolyte solution in this channel permeates the electrolyte matrix layer, so that the matrix layer is replenished. Since the bank portion is relatively small, the channel can store only a relatively small quantity of electrolyte solution. Thus, in the prior art fuel cells, as in the aforementioned case, the electrolyte solution cannot be resupplied to the level for a continuous operation for the commercial power generating time, 40,000 hours.
In replenishing the channel with the electrolyte solution, furthermore, the stacked structure must be disassembled. Therefore, the maintenance of the fuel cell requires much time.